Conventionally, biological wastewater treatments of organic matter-containing wastewater, which contains organic matter or the like, have used the activated sludge method that utilizes an aggregate of microbes known as a floc (aerobic biological sludge). However, in the activated sludge method, when separating the floc (aerobic biological sludge) and the treated water in a settling pond, because the settling rate of the floc is slow the surface area of the settling pond must sometimes be made extremely large. Further, the treatment rate of the activated sludge method depends on the sludge concentration inside the biological treatment tank, and while the treatment rate can be increased by increasing the sludge concentration, if the sludge concentration is increased to a range from 1,500 to 5,000 mg/L or even higher, then solid-liquid separation becomes difficult due to bulking or the like within the settling pond, and the treatment cannot be maintained in some cases.
On the other hand, anaerobic biological treatments generally utilize aggregates in a particulate form known as granules (anaerobic biological sludge), which contain densely aggregated microbes. Granules have an extremely fast settling rate, and because the microbes are densely aggregated, the sludge concentration inside the biological treatment tank can be increased, and a high-speed treatment of the wastewater can be achieved. However, anaerobic biological treatments can suffer problems, including restrictions on the types of wastewater that can be treated compared with aerobic treatments (activated sludge method), and a requirement to hold the temperature of the treated water at approximately 30 to 35° C. Further, if only an anaerobic biological treatment is used, then the water quality of the treated water is poor, and when the treated water is to be discharged into a river or the like, a separate aerobic treatment such as the activated sludge method may sometimes be required.
In recent years, it has become clear that by using a semi-batch reaction tank that repeatedly carries out an operation cycle that includes (1) a wastewater introduction step, (2) a biological treatment step of the treatment target substances, (3) a biological sludge settling step, and (4) a treated water discharge step, granules having good settling properties can be formed not only with anaerobic biological sludges, but also with aerobic biological sludges (for example, see Patent Documents 1 to 4). By forming granules having good settling properties as described above, the sludge concentration inside the tank can be maintained at a high concentration, thereby enabling a high-speed treatment.